Generally, in a single-wafer plasma processing apparatus, a susceptor (lower electrode) and a facing electrode (upper electrode) have diameters larger than that of a target substrate mounted on the susceptor in a vacuum chamber in order to improve in-plane uniformity of a density of plasma applied to the surface of the target substrate (especially, uniformity of the substrate in a radial direction). In this case, when a peripheral portion of a top surface of the susceptor which is projected outwardly in the radial direction of the substrate is directly exposed to the plasma, the peripheral portion is damaged and deteriorated by ion bombardment from the plasma.
Especially, in a plasma etching apparatus, since ions are accelerated and attracted by using a self-bias voltage generated in the susceptor, an ion sputtering effect is large. Accordingly, the susceptor is protected from the plasma by detachably providing a ring-shaped covering member, i.e., a so-called focus ring, so as to cover the peripheral portion of the top surface of the susceptor which is projected outwardly in the radial direction of the substrate (see, e.g., Japanese Patent Application Publication No. 2000-36490 and corresponding U.S. Pat. No. 6,676,804).
The focus ring is preferably made of a material that can efficiently pass a radio frequency (RF) power to the susceptor and the plasma and have no substantial influence on the process performed on the substrate even though it is sputtered by ions. For example, in a plasma etching apparatus, the focus ring is made of Si, SiC, C (carbon), SiO2, Al2O3 or the like.
Meanwhile, in the plasma processing apparatus, the temperature of the substrate has a large influence on a reaction on the surface of the substrate, etching characteristics, film characteristics and the like. Accordingly, the temperature of the substrate is controlled by a susceptor for mounting the substrate thereon. Generally, it is preferable to prevent the temperature of the substrate from being increased due to heat supplied from the plasma. Particularly, in a plasma etching process, a radical reaction is suppressed by decreasing the temperature of the substrate, thereby achieving high selectivity and vertical processability.
The temperature of the substrate is usually controlled by a cooling method in which a temperature-controlled coolant is supplied into and circulated along a coolant path provided in the susceptor from a chiller unit to cool the susceptor to a predetermined temperature, and a heat transfer gas such as He gas is supplied to a contact interface between the susceptor and the substrate to indirectly cool the substrate. The cooling method requires a holding unit for holding the substrate on the susceptor against a supply pressure of the heat transfer gas and an electrostatic chuck is widely used as the holding unit.
In a conventional plasma processing apparatus, the temperature of the substrate is controlled as described above, but an individual temperature control is not carried out on the focus ring. Accordingly, when the RF power is low, an ion flux is weak and the temperature of the focus ring has a temperature close to the temperature of the substrate which is mounted on the susceptor together with the focus ring.
However, when the temperature of the focus ring is low similarly to the temperature of the substrate, plasma process characteristics may be deteriorated. For example, in a photolithography, a multilayer resist process is widely used to improve miniaturization and resolution of a pattern. In the multilayer resist process, when etching of a bottom anti-reflection coating (BARC) film or an intermediate mask (mask process) is carried out, transfer accuracy, i.e., maintenance of the shape (prevention of damage and deformation) of a resist pattern provided at the top is regarded more important than the selectivity and vertical processability. Accordingly, the RF power used for plasma generation is set to be slightly low.
Further, when an RF power with a relatively low frequency is used to control ion attraction from the plasma to the substrate, the RF power for ion attraction is set to be still lower (extremely, 0 W). Accordingly, an amount of the heat supplied to the substrate from the plasma is small, and the temperature of the substrate is not very high, so that the temperature of the focus ring is low, similarly to the temperature of the substrate. However, in the etching process performed under the above-mentioned conditions, an etching rate in an edge portion of the substrate becomes relatively high compared to that in a central portion of the substrate, thereby causing nonuniform etching characteristics on the substrate.
As the results of experiments conducted to solve the above-mentioned problem, it has been found that an etching rate in an edge portion of the substrate is relatively suppressed and uniformity of etching characteristics is improved by increasing the temperature of the focus ring to be much higher than the temperature of the substrate.
However, in mass-produced apparatuses, installation of a heater for heating the focus ring at the susceptor is very difficult mechanically and causes an increase in the manufacturing cost. Further, the focus ring may be required to be cooled according to the process. For example, in the above-mentioned multilayer resist process, when a base film of a multilayer resist, i.e., a film to be processed, is etched after the mask process, the temperature of the focus ring is preferably decreased to obtain high selectivity and vertical processability.
As described above, a plasma processing apparatus for mass production requires a temperature controlling device capable of freely and easily controlling (particularly, increasing) the temperature of the focus ring.